This invention relates generally to thermal piston engines, and more particularly to structural and conceptional improvements that increase the efficiency of such engines.
The regenerative thermal engine of this invention combines unique components to achieve high efficiencies and low engine weights in compact, structurally and thermally integrated units. The primary object of this invention is to devise adiabatic engines which are capable of operating at high pressures and temperatures utilizing the total expansion of the generated gases without the size and weigh customarily associated with such engines. Further, the use of exotic materials such as ceramics which add to the expense and complexity of such engines is not necessary in the thermal engines devised, enabling a flexibility in the choice of competing materials for construction of highly efficient but low cost engines.
The superior characteristics of the piston engine have numerous application, with both the military and commercial applications in transport and power generation well known. Numerous developmental paths are available for reducing specific fuel consumption, and for reducing the size and weight of the engine. Many of these paths, however, lead to undersized power plants of high complexity and cost.
In designing a high temperature, adiabatic engine, major problems are involved in selection of materials and design of structures capable of withstanding both high temperatures and pressures. Formulation of systems that can effectively and fully utilize the expanded pressure spectrum without thermal losses, particularly those losses associated with cooling local zones of high temperature, is a major challenge.
In order to effectively utilize the runout thermal energy of the resulting working agent in a compact unit, it is necessary to integrate select components which can most efficiently operate under conditions of low, medium or high pressures. A complete utilization of the thermal energy developed in the combustion process can be accomplished only in the case of an effective harnessing of the total expansion of the combustion gases, from the highest pressure of the cycle of the lowest pressure of the ambient air, exhausting the working gases at the lowest temperature possible.
However, a super-long expansion in the cylinders of a reciprocating piston engine is possible only in very large engines with very low rotations. In such engines as the Sulzer and the Burmmeister and Wain naval engines, in which the ratio of stroke to bore reach 3-4, thermal efficiencies exceed 53%.
In the 720.degree. rotation of the crank shaft during the thermal cycle of a four stroke engine, the evolution of the pressure in the time of the intake, compression, combustion-expansion and exhaust, define various periods of low pressure, medium pressure, and high pressure. The low and medium pressure periods of the cycle cover 80%-90% of the cycle. Only 10%-20% of the cycle or 70% of the 720.degree. of the 720.degree. cycle rotation is associated with the high pressure period of final compression, combustion and initial expansion. Despite the very short duration of the high pressure period (10-20% of cycle time) engines are constructed to withstand this maximum pressure throughout the 720.degree. rotation cycle. The mass of metal and high strength structure is wasted during the rest of the cycle in which only medium and low pressure is encountered. As a result of this factor, actual engines are big, heavy, expensive and inefficient.
In a basic embodiment of an engine capable of effective utilization of the full spectrum of expansion pressures is an integrated rotary-reciprocal compound engine which develops an equivalent compression ratio to the long stroke engines described. The low and medium pressures are developed in the rotary component and include 40% of the cycle in a rotocompressor for compression and 40% in a rotoexpander for expansion. The high pressures are developed in final compression and initial expansion in the reciprocal piston component.
Conventional engines are limited in peak pressures to approximately 150 bars. This level establishes a practical limit for compression ratios including supercharged engines. Thermal efficiency rises with increases in the compression ratio, but the limited peak pressure for conventional engines limits thermal efficiency. Peak pressure is limited in principle by friction, particularly by friction forces associated with the side thrust of the piston against the cylinder liner from the angular oscillation of the connecting rod, and, inertia, particularly inertial forces associated with the increase in size and weight of moving parts designed to accommodate increased peak pressures. These adverse factors have in the past defined the limit of evolution of conventional engines.
The unique engine designs described herein include features that resolve the problems described and enable increased peak pressures to be achieved in compact lightweight engines.